The present invention relates to an optical low-pass filter for a digital camera.
Recently, digital cameras have become widely used instead of cameras using silver-salt films. A digital camera is provided with an image capturing element such as a CCD (Charge Coupled Device), which converts an optical image into an electronic image (i.e., electric image signals), and the electronic image is stored in a recording medium, such as a memory or a floppy disk, as image data. If the image capturing element is constructed such that a plurality of pixels are arranged in row and column directions with regularity, the following problem occurs. That is, if a spatial frequency of an optical image formed on the image capturing element is relatively high in comparison with a pitch of the pixels (i.e., a distance between the centers of adjacent pixels), moirxc3xa9 or false color appears in the captured image.
In order to avoid the above problem, conventionally, an optical low-pass filter (LPF) is inserted between an imaging lens and the image capturing element. The LPF removes the high-spatial-frequency components, and only lower-spatial-frequency components, which do not cause the moirxc3xa9 or the like, are incident on the image capturing element.
FIG. 9 shows an example of a conventionally used LPF 10xe2x80x2. The LPF 10xe2x80x2 includes, from a photographing lens side (left-hand side in FIG. 9), a horizontally separating birefringent plate 1, a depolarization plate 2 and a vertically separating birefringent plate 3. In FIG. 9, for the sake of clarification in description, the plates 1, 2 and 3 are shown as separated. However, the actual device is constructed such that the three birefringent plates 1, 2 and 3 are integrally formed. That is, the first through third birefringent plates are adhered with each other.
In the following description, it is assumed that the imaging element has a rectangular shape, and a direction parallel to a longer side of a rectangular imaging element will be referred to as a horizontal direction, and a direction parallel to a shorter side of the rectangular imaging element will be referred to as a vertical direction. In FIG. 9, the horizontal direction and the vertical direction are indicated by arrows X and Y, respectively.
In general, a birefringent plate separates an incident ray into an ordinary ray and an extraordinary ray. The ordinary ray and the extraordinary ray are linearly polarized rays, whose oscillating directions (i.e., the polarized directions) are orthogonal to each other. It is known that a separation width, i.e., a distance between the ordinary ray and the extraordinary ray, are proportional to the thickness of the birefringent plate. For example, when the birefringent plate is formed of an artificial crystal plate, the separation width d and the thickness t of the birefringent plate have a relationship expressed by equation (1).                     d        =                                                                              n                  e                  2                                -                                  n                  o                  2                                                            2                ⁢                                  n                  o                                ⁢                                  n                  e                                                      ⁢            t                    =                      5.9            xc3x97                          10                              -                3                                      ⁢            t                                              (        1        )            
where, no is a refractive index for the ordinary ray, and ne represents a refractive index for the extraordinary ray.
In the conventional LPF 10xe2x80x2, the incident ray is separated by the horizontally separating birefringent plate 1 at a predetermined separation width in the horizontal direction. The rays emerged from the horizontally separating birefringent plate 1 are depolarized by the depolarization plate 2. Thus, the rays passed through the depolarized plate 2 oscillate similarly to natural light (i.e, unpolarized).
The two rays emerged from the depolarization plate 2 are incident on the vertically separating birefringent plate 3. Then, each ray is separated in the vertical direction by the vertically separating birefringent plate 3. Thus, a ray incident on the LPF 10xe2x80x2 is separated into four rays by the LPF 10xe2x80x2, and the rays emerged therefrom are incident on the imaging element. With this configuration, an image can be blurred, and the high-spatial-frequency components of the optical image can be removed.
If the ray incident on the LPF 10xe2x80x2 is not polarized in a particular direction (i.e., unpolarized), as the rays of the normal light, energies of the rays separated by the LPF 10xe2x80x2 and incident on respective pixels of the imaging element are substantially even. Therefore, the high frequency components can be removed evenly in both the horizontal and vertical directions.
Recently, a single lens reflex type digital camera (hereinafter, referred to as an SLR digital camera) has been developed and used widely. In some SLR digital cameras, in order to direct light passed through a photographing lens to the imaging element and a finder optical system, a beam splitter is provided between the photographing lens and the LPF.
Generally, the beam splitter is provided with a light separating coating, and light separated by the beam splitter has a certain polarization characteristics. Accordingly, a ray passed through/reflected by the beam splitter may not be separated evenly by the conventional LPF constructed as above. That is, in general, the energy of the vertically polarized component and the energy of the horizontally polarized component of the beam passed through/reflected by the beam splitter may be different.
If such a beam is incident on the conventional LPF 10xe2x80x2, the energies of the rays separated by the LPF 10xe2x80x2 may not be even. In a particular case, the energy of one or some of the separated rays may be substantially zero. In such a condition, the blurring effect may be different in the horizontal direction and in the vertical direction. Then, the high-spatial-frequency components may not be removed or suppressed sufficiently depending on the direction, and the false color phenomenon may remain. Therefore, the conventional LPF is not suitable to the SLR digital cameras employing a beam splitter between the photographing lens and the LPF.
It is therefore an object of the invention to provide an improved LPF (low-pass filter) which is capable of separating incident ray to a plurality of rays having substantially the same energy, even if the incident ray is polarized so that the high-spatial-frequency components of the optical image can be removed.
For the above objects, according to the invention, there is provided an optical low-pass filter used in association with an imaging element which has a plurality of pixels arranged in vertical and horizontal direction with regularity, is provided with first through third birefringent plates arranged in the order from a light incident side. The low-pass filter separates an incident ray into four rays, a separation width of the rays in the vertical direction being xcex4v, and a separation width of the rays in the horizontal direction being xcex4h. The separation angles of the first through third birefringent plates are xcex8(xcex8xe2x88x9290xc2x0) and 0xc2x0, respectively, and separation widths of the first through third birefringent plates are (xcex4vxc3x97sin xcex8), (xcex4vxc3x97cos xcex8) and xcex4h, respectively, xcex8 being greater than 0xc2x0 and less than 90xc2x0.
In a particular case in the above configuration, the separation angle xcex8 may be substantially 45 degrees.
Alternatively, the separation angles of the first through third birefringent plates are xcex8, (xcex8+90xc2x0) and 0xc2x0, respectively, and the separation widths of the first through third birefringent plates are |xcex4vxc3x97sin xcex8|, xcex4vxc3x97cos xcex8 and xcex4h, respectively, xcex8 being greater than xe2x88x9290xc2x0 and less than 0xc2x0.
In a particular case in the above configuration, the separation angle e may be substantially xe2x88x9245 degrees.
Further alternatively, the separation angles of the first through third birefringent plates, with respect to the horizontal direction, are xcex8, (xcex8+90xc2x0) and 90xc2x0, respectively, and the separation widths of the first through third birefringent plates are (xcex4vxc3x97cos xcex8), (xcex4vxc3x97sin xcex8) and xcex4v, respectively, xcex8 being greater than 0xc2x0 and less than 90xc2x0.
In a particular case in the above configuration, the separation angle xcex8 may be substantially 45 degrees.
Still alternatively, the separation angles of the first through third birefringent plates, with respect to the horizontal direction, are xcex8, (xcex8xe2x88x9290xc2x0) and 90xc2x0, respectively, and the separation widths of the first through third birefringent plates are |xcex4hxc3x97cos xcex8|, (xcex4hxc3x97sin xcex8) and xcex4v, respectively, xcex8 being greater than 90xc2x0 and less than 180xc2x0.
In a particular case in the above configuration, the separation angle xcex8 may be substantially 135 degrees.
Optionally, the ray incident on the first birefringent plate may have different energies in the horizontal direction and in the vertical direction.
Further optionally, the first, second and third birefringent plates may be integrally formed.
Furthermore, at least one of a light incident surface of the first birefringent plate and a light emerging surface of the third birefringent plate may be formed with an infrared cut coating.
Optionally, a dummy glass element may be integrally formed with the integrally formed first, second and third birefringent plates.
In this case, a thickness of said dummy glass element may be adjusted so that the optical low-pass filter has a predetermined thickness regardless of the thickness of each of the first, second and third birefringent plates.
In a particular case, the separation widths xcex4h and xcex4v are substantially the same.